Shortening the Time Between Phases – Follow-up

Before I started to shorten the time between firing phases, I did not have a chance to check the pieces after the first phase, since I had never taken them out of the carbon. Now that I have done it quite a few times, I can tell that there is an important difference between the clays at this point (after the first phase), and if you are not aware of this difference, you may end up destroying your piece.

Bronze and White Bronze are quite stable after the first phase of firing, and if you handle them carefully (light touch, no tweezers), they should be fine. This is not the case with copper, and even less so with steel. These two metals are so fragile at this stage that they may break as a result of even the slightest tilt of the box when you take it out of the kiln (not to mention the effect of actually touching them).

The first phase is meant to burn away the binder. At the end of this phase, there is actually nothing to hold the metal particles together. The removal of the binder allows the particles to get closer in the second phase, until they actually bond and become a solid mass. This is why the pieces are so fragile at the end of the first phase. It seem a miracle that the do stay together.

For now, I would like to avoid discussing why bronze and White Bronze are less fragile after the first phase. I would be glad to discuss this in actual workshops. The point is that you need to be extra careful with copper and steel, and if you don’t trust yourself, just take the box out of the kiln carefully and let it cool naturally. It is recommended to use rigid boxes that don’t wobble, as a ceramic blanket is likely to do – you can use a fiber board box, or even a mixing bowl (just remember to adjust the temperature for each type of box).

Firing Steel and Copper

If your piece is constructed of steel or copper, or a combination of steel and copper, with no bronze or White Bronze in it, fire at 1650°F/900°C, as the updated Instruction Manual suggests. They will not be very strong at a lower firing temperature.

When we fire them with bronze and White Bronze, we are actually under-firing them since bronze and White Bronze will not tolerate such high temperatures. In that case, it’s the bronze or White Bronze that give the piece its strength.

To quote the unforgettable Sgt. Phil Esterhaus of Hill Street Blues: “Let’s be careful out there!”

This entry was posted on Wednesday, March 2nd, 2011 at 2:40 pm. You can follow any responses to this entry through the RSS 2.0 feed.
You can leave a response, or trackback from your own site.

Metal Clay Supply just sent an e-mail about their new ceramic boxes for firing metal clay. They are tall (3 3/4″). Could they solve the problem of the mini-kilns lacking room for enough carbon? I assume that you would fire them with the lid off? If these could work, it would be worth the cost as it saves me the labor of cutting the firebrick and glueing it together, not to mention avoiding the dust. Your input would be much appreciated!

Out of curiosity, what is the reason for requiring a cool down at all between firings? I am sure there is one but I can’t wrap my pointy little head around what it might be.
Thank you SO much for your wonderful experimental approach and your incredible willingness to share! You are amazing!

Cooling and re-firing gives any leftover binder that did no burn a chance to burn off completely. As the temperature rises, there is less and less oxygen available for it to burn. Without burning all the binder, there will be no sintering. Does that make sense?

Thanks so much for this posting. I just got the new ceramic box that has recently become availoable and I am doing all my test strips and I realized that I can do the first phase with all 4 types of metal test strips and then carefully remove the white bronze and bronze for firing in the second phase later and continue with the steel and copper. I have been having good luck so far with the new container, but the firing temps need to be a bit higher than with the fiber blanket. The nice thing about the ceramic is it cools very quickly. I wish I had though of this earlier but it will save me quite a few hours in my final testing stages.

(this is in response to Wendy’s question and your answer)So if I understand correctly, the purpose of the cool down is to allow more oxygen to get to the pieces. If the Carbon is too hot, it will continue to reduce the oxygen too much. Is that right?